The involvement of tumor growth factor-beta (TGF-beta) in embryonal development of the mouse was investigated in embryos of 10 to 18 days of gestation, using antibodies raised against synthetic peptides of the TGF-beta monomer to localize the growth factor. TGF-beta was found in a variety of tissues of ectodermal and mesenchymal origin, predominantly around day 15 when organogenesis is most intense. The wide distribution of TGF-beta indicates its involvement as a regulator in major events of cytodifferentiation.